Core diameter mismatch structures are proposed and experimentally investigated for curvature and vibration sensing. Two configurations are suggested, one approach uses a structure formed by splicing an uncoated short section of multimode fiber between two standard single-mode fibers (SMFs) single-mode-multimode-single-mode (SMS), combined to a fiber optical mirror at its end, and the other approach uses a structure made by splicing a section of SMF between two multimode fibers (SMSMS). In the curvature analysis, the proposed SMS sensor generates the destructive interference patterns when it is bent, varying only the attenuation of the optical signal without wavelength shifts. The SMSMS vibration sensor proved to be suitable to monitor very low frequencies such as 0.1 Hz. The configuration of the proposed sensors presents several interesting features, such as easy fabrication, low cost, high efficiency, and high sensitivity. These advantages make such sensors very useful in a wide range of applications, for instance, structural health monitoring.

In this article, we proposed a simple optical sensor for flow measurement. The sensor structure is based on an in-line Mach-Zehnder interferometer formed by splicing an input single-mode fiber section and an output single-mode fiber section to a single-mode fiber sandwiched by two multimode fibers; it is called SMSMS sensor. The proposed sensor presents a high sensitivity due to low order modes that propagate in the middle single-mode fiber (SMF) cladding. The sensor is experimentally investigated and validated in which the results show good agreement between the optical power and flow for monitoring single- and two-phase flows using water and air fluids. Using this sensor, it is possible to identify the presence of air in the pipeline, in addition to the identification of the flow pattern. Thus, the proposed sensor is an interesting alternative for measuring single- and two-phase flows.